Rechargeable batteries are widely used power sources, for example in calculators, computers, appliances and automobiles. Batteries consisting of solid state rechargeable electrochemical cells are of great interest because they allow important reductions in size and weight compared with the more traditional type of battery.
Solid state rechargeable electrochemical cells are well known in the art. Typically, these cells are constructed in layers composed of an alkali metal foil anode (negative electrode), an ionically conducting solid polymeric electrolyte separator, and a composite cathode (positive electrode). Terminals are attached to the anode and cathode thus forming an electrochemical cell. The cell may be sealed in a gas and liquid impervious packaging material from which the terminals protrude. There is no free flowing liquid in this type of cell. These solid state cells are of great commercial and technical interest. Use of solid state rechargeable electrochemical cells results in important weight reductions for example in automobiles, thus leading to improved automobile fuel efficiency.
Anodes suitable for use in solid electrochemical cells are usually formed from an alkali metal sheet or foil. Typically, the alkali metal of choice is lithium because of its low density and highly electropositive nature. See, e.g., U.S. Pat. No. 4,748,542 (Lundsgaard, 1988). The use of lithium-coated foil such as nickel or copper foil having a layer of lithium deposited on its surface, or the use of a lithium alloy is disclosed by U.S. Pat. No. 4,935,317 (Fauteux et al., 1990).
A solid ionically conducting electrolyte forms the separator between the cathode and the anode. Lundsgaard '542 teaches that the separator should be ionically conductive but electronically non-conductive. Typically, the separator comprises an ionically conductive polymer matrix similar to those described for use in the composite cathode. Most commonly, the separator is composed of a polymer such as polyethylene oxide and ionizable alkali metal salt in an aprotic solvent, see for example Fauteux '317. Alternately, the solid electrolyte separator may be composed of solvent ligands and polymer ligands coordinated with metal ions, see U.S. Pat. No. 5,229,225 (Shackle, 1993). A layer of solid electrolyte separator is deposited on the composite cathode layer, and is then polymerized.
Fauteux '317, discloses a typical composite cathode composition including active cathode materials such as transition metal chalcogenides or intercalation compounds, electronically conductive particles, an ionically conductive polymer matrix and a current collector. Representative examples of transition metal oxides and sulfides useful as active cathode materials are: V.sub.6 O.sub.13, V.sub.2 O.sub.5, MoO.sub.2, TiS.sub.2, MnO.sub.2, MoS.sub.3, Cr.sub.3 O.sub.6, Li.sub.x V.sub.3 O.sub.8, FeS, NiS, CoO and CuO. Fauteux '317 discloses that V.sub.6 O.sub.13 is a particularly preferred intercalation compound. Useful electronically conductive particles are: conductive carbon black particles and certain conductive polymers which are characterized by a conjugated network of double bonds (e.g., polypyrrole and polyacetylene). U.S. Pat. No. 4,303,748 (Armand et al., 1981) teaches the use of graphite to obtain electronic conductivity in the composite cathode.
The cathode intercalation compound and the electronically conductive particles are mixed with a monomer to form a paste which is polymerizable by radiation or heat to form a polymeric network. The polymer network serves to hold the solid particles; it also serves to provide ionic conductivity in the cathode. Ionic conductivity of the polymer matrix may be obtained by using an ionically conductive liquid which forms an interpenetrating conductive phase within a conductive or a non-conductive polymer matrix.
Conductive polymers are well known in the art. For example, U.S. Pat. No. 4,990,413 (Lee et al., 1991) discloses ionically conductive polymers suitable for cathode compositions. Lee '413 teaches that these polymers have repeating units containing at least one and preferably a plurality of heteroatoms particularly oxygen and/or nitrogen and which are preferably terminated by radiation polymerizable moieties. Solid electrolyte compositions comprising a polymeric network interpenetrated by an ionically conducting liquid phase are well known in the art, see e.g., the following U.S. Pat. No. 5,229,225 (Shackle, 1993); U.S. Pat. No. 5,037,712 (Shackle et al., 1991); U.S. Pat. No. 4,990,413 (Lee et al., 1991); U.S. Pat. No. 4,830,939 (Lee et al., 1989); and U.S. Pat. No. 4,792,504 (Schwab et al., 1988). Typically, these solid electrolyte compositions contain a solution of a conductive salt (such as certain lithium, sodium or potassium salts) in an aprotic solvent which forms a continuous phase in a crosslinked polymer such as polyethylene oxide.
The composite cathode is formed by depositing a layer of the mixture comprising intercalation compound and electronically conductive filler in ionically conductive monomer, on a current collector. The monomer is then heat or radiation polymerized, forming a bond with the current collector. Cathode current collectors are well known in the art, they typically comprise a metal foil. Fauteux '317 teaches the use of aluminum, nickel or stainless steel, and the process of forming the composite electrode.
As mentioned above, vanadium oxide V.sub.6 O.sub.13 is commonly used as an intercalcation compound in cathodes of solid state rechargeable electrochemical cells, see for example U.S. Pat. No. 5,229,225 (Shackle, 1993); U.S. Pat. No. 5,219,680 (Fauteux, 1993); U.S. Pat. No. 5,217,827 (Fauteux, 1993); U.S. Pat. No. 5,037,712 (Shackle et al., 1991); U.S. Pat. No. 4,997,732 (Austin et al., 1991); U.S. Pat. No. 4,990,413 (Lee et al., 1991); U.S. Pat. No. 4,935,317 (Fauteux et al., 1990); and U.S. Pat. No. 4,830,939 (Lee et al., 1989).
Application of the anode to the electrolytic separator completes the laminar construction of a cell consisting of alternating layers of cathode, electrolytic separator and anode. Terminals are secured to the anode and the cathode to provide for electrical contact.
The synthesis of vanadium oxide V.sub.6 O.sub.13 by thermal decomposition of ammonium vanadates is well known in the art. Examples of this synthesis are provided in U.S. Pat. No. 4,035,476 (Ilmaier et al., 1977) and U.S. Pat. No. 4,486,400 (Riley, 1984). Ilmaier '476 discloses a process for making V.sub.2 O.sub.x, wherein X is between 3.8 and 4.6 (corresponding to V.sub.6 O.sub.11.4-13.8), by thermally decomposing ammonium poly(hexa)vanadate at 600.degree.-900.degree. C. in the presence of an inert gas. The Ilmaier process results in a sintered product.
Riley '400 discloses the synthesis of V.sub.6 O.sub.13 usable as the active material in cathodes used in high energy density cells. Riley decomposes ammonium metavanadate NH.sub.4 VO.sub.3 by gradually heating NH.sub.4 VO.sub.3 in a stepped process from 350.degree. to 400.degree. C. for about 6 hours, followed by heating at 400.degree. to 500.degree. C. over a period of 8-12 hours. The initial part of the heating is carried out in a nitrogen flow, followed by heating in a gaseous mixture having an oxygen partial pressure equal to the V.sub.6 O.sub.13 oxygen partial pressure. Suitable gaseous mixtures are carbon monoxide/carbon dioxide and hydrogen/water vapor. Riley produces V.sub.6 O.sub.13 having an average particle size of about 2 microns and a surface area of about 17 m.sup.2 /g.
Pryor et al., "Large Scale Preparation of Non-Stoichiometric V.sub.6 O.sub.13," Preprint from the 16th International Power Source Symposium, 1988, discloses the effects of various processing conditions on the properties of V.sub.6 O.sub.13 synthesized by thermal decomposition of NH.sub.4 VO.sub.3 in an argon gas flow. Heating was carried out in a rotating drum furnace containing NH.sub.4 VO.sub.3 and ceramic grinding media. The material was heated at a rate of 1.8.degree. C./min. to a final temperature of about 427.degree. C., it was then held at this temperature for four hours prior to cooling the product. Argon flow was continued throughout the heating and cooling process. Variations in reaction conditions resulted in VO.sub.x wherein the stoichiometric ratio X ranged from 2.18 to 2.24 (corresponding to V.sub.6 O.sub.13.08-13.44). The BET surface area ranged from 10.0 to 14.6 m.sup.2 /g. Particles obtained by Pryor et al. (1988) consist of loosely bound agglomerates which may be reduced to single crystallites when dispersed with surfactant. "BET surface area" refers to the method developed by Brunnauer, Emmett, and Teller for calculating surface area based on gas adsorption. See, e.g., Martin, Swarbrick, and Cammarata, Physical Pharmacy, 3rd Edition, p. 508 (Lea & Febiger, Philadelphia, 1983). Pryor's Scanning Electron Microscopy ("SEM") data show V.sub.6 O.sub.13 agglomerates.
The prior attempts to synthesize V.sub.6 O.sub.13 have not produced a composition with a physical structure more nearly optimal for use in a composite cathode. Accordingly, the need exists for an improved V.sub.6 O.sub.13 which, because of its physical shape or structure, has enhanced performance in rechargeable electrochemical cells thus leading to further reductions in cell weight and size.